In general, a wireless communication system performs a communication using a wireless signal of a predetermined band between a terminal and a base station. The wireless communication system includes a mobile communication system using a mobile phone, a wireless broadband (Wibro) communication system, a satellite broadcasting system, and a terrestrial broadcasting system. As mentioned above, the wireless communication systems transmit data between a terminal and a base station through a predetermined radio frequency.
However, as a frequency is getting higher, a radio frequency has a characteristic that refraction and diffraction do not occur. A signal transmitted from a base station has a limitation of a signal transmission distance by a used frequency and topographical condition, that is, topography and building. Accordingly, a designer who designs a wireless communication system must install a base station based on a limitation of a signal transmission distance.
Here, there is a problem in that a lot of base stations must be installed in a crowded building district, or in a case that an area covered by one base station is widened. That is, in case that an area covered by one base station is widened, or a lot of transmitters are installed in a small area, a wireless communication service provider has a lot of loss. Accordingly, it is proposed to transmit a signal using a main transmitter and a repeater as a method for reducing the number of transmitters and transmitting data efficiently. There is a method for coupling the main transmitter with the repeater in a wire communication and a wireless communication.
Firstly, in case of using a wire communication between the main transmitter and the repeater, a plurality of wireless repeaters are used in a small area. However, because it is difficult to use the wire communication if the main transmitter is far from the repeater, a distance limitation can be solved by transmitting data from the main transmitter to the repeater using a radio frequency.
It will be described in detail to transmit data between the main transmitter and the repeater using a radio frequency referring to the accompanying drawings.
FIG. 1 is a simplified diagram illustrating a method for transmitting data between a main transmitter and a repeater using a different frequency.
A main transmitter 101 transmits a signal to repeaters 111, 112, 113 and 114 using a frequency A. The repeaters 111, 112 113 and 114 receive a signal of the frequency A, and re-transmit a signal to an area of the repeaters using different frequencies which are used in each of the repeaters. That is, a first repeater 111 transmits a signal to an area of the first repeater 111 using a frequency B, and a second repeater 112 transmits a signal to an area of the second repeater 112 using a frequency C. A third repeater 113 transmits a signal to an area of the third repeater 113 using a frequency D, and a fourth repeater 114 transmits a signal to an area of the fourth repeater 114 using a frequency E.
Finally, the main transmitter 101 transmits a signal to repeaters 111, 112, 113 and 114 which are neighbored to the main transmitter 101 using a frequency A, and the repeaters 111, 112, 113 and 114 transmit a signal to the area of the repeaters using an allocated frequency. Data can be transmitted without a shadow area by using the transmission method mentioned above. That is, it is possible to remove an area where a signal is unstably received and increase a service area.
However, as shown in FIG. 1, if the main transmitter 101 and repeaters 111, 112, 113 and 114 use different frequencies, they need a wide frequency band. This causes the deterioration of the reuse efficiency of a frequency which is a limited resource. Thus, it is necessary to improve the reuse efficiency of a frequency.
FIG. 2 is a simplified diagram illustrating a method for transmitting data between a main transmitter and repeaters using the same frequency.
As shown in FIG. 2, a main transmitter 201 and repeaters 211, 212, 213 and 214 transmit a signal using the same frequency A. That is, the main transmitter 201 transmits a signal to an area of the main transmitter 201 using the frequency A. The repeaters 211, 212, 213 and 214 receive the signal transmitted from the main transmitter using the frequency. If the repeaters 211, 212, 213 and 214 receive the signal of the frequency A, the repeaters 211, 212, 213 and 214 transmit the received signal using the same frequency A. Accordingly, the repeaters 211, 212, 213 and 214 maximize the reuse efficiency of the frequency by receiving the signal of the frequency A and transmitting the received signal to an area of the repeaters using the frequency A.
However, as shown in FIG. 2, if the main transmitter 201 and the repeaters 211, 212, 213 and 214 transmit a signal using the same frequency, the high isolation of a transmission/reception antenna between the main transmitter 201 and each of the repeaters 211, 212, 213 and 214 is indispensable. To satisfy the high isolation of the transmission/reception antenna, the main transmitter 201 and each of the repeaters 211, 212, 213 and 214 must be exchanged. That is, if the main transmitter 201 and the repeaters 211, 212, 213 and 214 use the same frequency, the utility of a transmission device used in a conventional wireless communication system is lowered, it needs a new facility investment.
FIG. 3 is a simplified diagram illustrating a method for transmitting data by configuring a distributed repeater network using a distributed repeater.
FIG. 3 shows a distributed repeater network using a distributed repeater which combines the method shown in FIG. 1 and the method shown in FIG. 2. In the method shown in FIG. 3, a main transmitter 301 transmits an area of the main transmitter including repeaters 311, 312, 313 and 314 using a frequency A. The repeaters 311, 312, 313 and 314 receive the signal of the frequency A from the main transmitter 301, and transmit a signal to an area of the repeaters 311, 312, 313 and 314 using a different frequency B. Compared with the method shown in FIG. 1, the method shown in FIG. 3 improves a reuse efficiency of a frequency. Moreover, compared with the method shown in FIG. 2, the method shown in FIG. 3 solves the isolation of a frequency.
However, if the repeaters 311, 312, 313 and 314 are neighbored, the interference of signals can occur. In case of FIG. 2, if the repeaters 211, 212, 213 and 214 are neighbored, and each of the repeaters 211, 212, 213 and 214 and the main transmitter 201 are neighbored, the interference of signals can occur.
To solve the interference problem of signals between the main transmitter and each of the repeaters, or between the repeaters, it is proposed to allocate an identification signal having an excellent correlation characteristic to each of the repeaters and/or the main transmitter, add and transmit the allocated identification signal to a transmission signal and a repeating signal.
In general, a sequence used as an identification signal is embedded as a spread spectrum shape to minimize an influence of a conventional service signal. Accordingly, the identification signal needs a high bit resolution to identify transmitters and/or each of repeaters. Moreover, a long sequence is used for an excellent correlation characteristic. For example, in a USA type digital broadcasting, that is, Advanced Television Systems committee (ATSC) system, a Kasami sequence of 64,896 length is used as an identification signal. The identification signal is inserted to have 21 dB-39 dB less than a broadcasting service signal power of an ATSC type digital broadcasting.
As described above, if the identification signal is used, although the interference signal exists, a wanted signal can be detected. But, if a system is configured as described above, the identification signal must be detected and analyzed. Thus, to detect and analyze the identification signal, it is necessary for a lot of calculation quantity and high complexity. Moreover, a separation and an attenuation phenomenon of an extracted channel profile must be compensated.
Furthermore, it is indispensable that a service provider for implementing a system recognizes correctly an intensity of a signal received from repeaters and an intensity of a signal received from a main transmitter at a specific place. Because it is possible to adjust a power or add the repeaters or the main transmitter if the signal intensity from the transmitter is recognized when determining an additional installation of the system or the increase/decrease of the power. Accordingly, it is necessary to correctly measure the intensity of the signal received from the transmitter.